Add Visibles BDT to ldmx-sw

[horohoo]

Is your feature request related to a problem? Please describe.
This is a request to add the BDT for the Visibles analysis recently described in a SWAN meeting.

Describe the solution you'd like
I propose three things be added to ldmx-sw, all in the Hcal branch:

• A Python script for training the BDT.
• A new VisiblesVetoProcessor class.
• BDT weights in .onnx format.

Describe alternatives you've considered
There is no feasible alternative. This analysis tool should be made easily available to the whole collaboration.